Primary controlled switching power supplies are widely used in applications, such as LED drivers, chargers, adapters, and so on, due to fewer components, a simplified structure, and lower product costs. As shown in FIG. 1, an output voltage may be sampled by detecting a voltage across an auxiliary winding of a transformer. Such a primary controlled system may sample the output voltage by detecting a voltage across the auxiliary winding of the transformer in every switching cycle, instead of directly detecting the output voltage. Thus, drawbacks of this approach may be in failing to timely obtain the output voltage change information when an output load switches from a light-load or no-load state to a heavy-load or full-load state, resulting in relatively poor dynamic performance. Also, the system frequency may be unable to be further decreased due to dynamic response characteristics of the primary controlled system, which can limit the decrease of power losses.